"Unless you plan on going FF anytime soon, look at the 17-55 f/2.8 IS. I just bought one for my 7D, and it's very good. It will probably be on the camera 90% of the time. "

I agree. 24mm narrows to about 40mm on a crop body; the 17 - 55 or Sigma 17 - 50 are designed specifically to replicate the "standard" range on smaller sensors. I, for example, just got the Sigma, chiefly because it's $500 less than the Canon, and the latter offers at most marginal improvements.